High-purity argon is produced from air by cryogenic distillation; the process is energy intensive because of the similar volatilities of oxygen and argon. This work investigates the potential to reduce the energy consumption of argon production by using membrane-assisted distillation in the crude argon column of an air separation unit (ASU). Membrane-assisted distillation flowsheets are developed and simulated in Aspen Plus®. A customised model for the membrane separation is implemented in Aspen Plus as a user-defined unit. The built-in optimisation tool in Aspen Plus is used to optimise process operating conditions and the location of the membrane unit along the distillation column with the objective of minimising the overall power demand of the process. The performance of ambient polymeric membrane separations and low-temperature carbon molecular sieve membranes separations is evaluated.
Detailed simulation and optimisation results show that membrane-assisted distillation offers considerable power savings when the membrane is placed in parallel to the distillation column. For both types of membrane material, the reduction in specific power demand (i.e. per unit of argon produced) is greatest when the membrane is placed close to the feed stage of the distillation column. A reduction in specific power demand of 12 %, relative to conventional distillation, is found for commercially available polymeric membranes. The decrease in specific power demand would be even greater (up to 32 %), if carbon molecular sieve membranes operating at low temperatures could be used.